The present invention relates to a laser-plasma generation method and its structure in which a target material that is fluid at least in one of liquid and solid states is ejected from a nozzle to form a column-like jet flow and then irradiated by focused pulsed lasers composed of at least single beam to generate a high-temperature high-density plasma that emits high-average power x-ray which is useful for x-ray material processing, x-ray lithography, and material analysis. In particular, the present invention relates to a laser-plasma generation method and its structure which provides a method that can supply with target continuously at a small gas emission rate, and which can enhance the x-ray conversion efficiency.
The effort has been made to realize a practical laser-plasma x-ray source in order to use x-rays from a high-temperature plasma that is produced from a target and heated by irradiating a high peak power laser. It is desirable to obtain an x-ray conversion efficiency as high as possible in the focus spot area of the heating laser beam in order to achieve a high-brightness x-ray source. On the other hand, a higher plasma density is favorable to obtain a higher x-ray conversion efficiency. For this purpose a method has been developed as a mean supplying the target that prepares the target material in a fluid state of a gas phase or a liquid phase which is ejected as a jet flow through a nozzle into a vacuum chamber and then heated by a focused laser beam. With this method, a continuous supply of the target material is relatively easy, thus a high average power x-ray source will be achieved by using a high repetition rate pulsed laser.
Use of a supersonic gas jet flow has been popular as the ejected jet flow. However, since this jet flow is a gas flow a number density of target molecules at the upper stream of the nozzle is smaller in two orders of magnitude than a solid state density and a divergence angle of the ejected flow is as large as 30 angular degree, thus lowering a target density significantly at the laser focus spot which is well separated from the nozzle exit. This results in a degraded x-ray conversion efficiency compared to the case when using the target of solid state where an x-ray conversion efficiency is defined as a fraction of the laser energy converting to an energy of emitting x-rays. Therefore it is being tried to generate higher density plasmas by placing the laser focal spot several millimeters down from the nozzle exit. But this causes erosion of the nozzle metal surface, resulting in emission of metal debris from the nozzle.
In order to solve this problem a method was proposed to produce a jet flow consisting of molecular clusters that is formed by ejecting the target gas which is cooled down at the upper stream of the nozzle, thus being condensed to a high density very close to liquid density. With this method it is expected that the target density is enhanced by molecular clustering at the nozzle exit and a smaller divergence of the flow. But the divergence angle does not become small enough in practice. For example, when the laser focus spot is placed 10 mm down the nozzle exit, the ejected flow is in a state of spray so that the density of the target molecules is not high enough to obtain a required x-ray intensity.
On the other hand, compared with the supersonic gas jet flow above described, a droplet target guarantees a target of liquid density at the laser focus spot with a minimum mass of the target material. However, there are serious problems regarding stabilities in their size and trajectories.
Furthermore a high-velocity and a precise synchronization with the laser pulse in time are required for the droplet target to be operated at a high repetition rate because the distance between one droplet and the next one has to be large enough not to destruct the next one due to plasma particles and/or scattered laser beam, and the focused laser beam has to hit the droplet center precisely in time and space.
Thus, use of a continuous high-velocity liquid jet flow would be a most straight-forward method to solve the above problems. For example, in the method disclosed in a Japanese patent application file (JP-A) 2000-509190, a target generation mean 1 ejects a liquid continuously, generating a jet-flow target 2 as shown in FIG. 1. A laser beam 3 is focused and irradiated on a focus spot 4 that is placed at a spatially continuous part of the jet-flow target 2, thereby ionizing the jet-flow target 2 to produce a plasma that emits x-rays.
On the other hand, it has been proposed to use low-temperature Xenon for a liquid-jet laser-plasma x-ray source, for example by B. A. M. Hansson et al (In Emerging Lithographic Technologies IV, Proceedings of SPIE Vol.3997, 2000). According to their report, a diameter of the liquid Xe jet flow is limited in less than 40 μm because of evacuation capacity of their vacuum pump and likely because of hydrodynamic instability, and the position of the focus spot needs to be confined only in the range of continuous liquid flow.
In solid droplet or liquid jet targets, laser heating of the target induces a strong pressure impulse that drives a shockwave heating inside the target, while only radiation outgoing from the ablation plasma heated by the laser pulse is being used as the x-ray source. On the other hand, the hydrodynamic energy carried by shockwaves, compression waves etc. is dissipated in the target, causing the debris emission and/or massive evaporation in the surrounding target material.
Enhancement of the x-ray conversion efficiency has been tested by using cavity-structured targets. A pulsed laser beam was introduced through an inlet hole to irradiate the cavity inside surface. The cavity structure would confine at least one of x-rays and plasma particles produced by laser heating in the cavity. The elongation of x-ray emission pulse duration was observed and some enhancement of the brightness in the emission from the hole was confirmed.
As is above described, a droplet target can provides a liquid or solid density at the laser focus spot so that a higher x-ray conversion efficiency is obtained compared to gas jet targets. However, it is not easy to control their sizes, trajectories, velocities and repetition rate in a stable manner. Furthermore it is doubtful to use droplet targets at a high repetition rate operation because a high velocity and a precise timing of the droplets are required as above described
On the other hand, in the case of a liquid jet target the mass of target material consumed can be reduced by reducing a diameter of the column-like jet flow. But the diameter of the jet needs to be as large as the focus spot diameter of the laser in order to obtain a sufficient x-ray conversion efficiency. Otherwise, the laser beam interacts with the under-dense region of an expanding plasma so that the laser energy is not well absorbed and the ion density thereof is relatively low. Thus it tends to generate relatively lower intensity emission. Accordingly, the x-ray emission intensity in this case is less than expected from consumption of target material compared to the case of droplet target. This means that a much larger quantity of target gas is generated in a vacuum chamber for x-ray source than expected from the x-ray power obtained.
In cavity target, an overall x-ray conversion efficiency might be enhanced because the laser energy absorption rate is improved in the cavity plasma of a relatively large scale-length, and the energy of the plasma particles confined in the cavity will be converted into the radiation energy during a confinement time. But, in addition to the difficulty in supplying the cavities at a fast rate the x-ray flux emitted from the outlet hole of the cavity is much limited by the size of the hole. The x-ray energy available through the cavity hole is very little compared to the x-ray energy which will be dissipated in heating and ablating the inside wall of the cavity. The ablation front driven by x-rays proceeds into the cavity wall, ionizing the wall material, thereby dissipating its energy.
When the target gas generated is not well evacuated, the pressure in the vacuum chamber rises so that the x-ray emitted from the target plasma is absorbed by neutral gas molecules of target material which stay at the region surrounding the source point. Therefore the x-ray intensity which is usable outside the chamber is attenuated. We need vacuum pumps of huge evacuation capacity to avoid the above attenuation.
The purpose of the present invention is to provide with a method of laser-plasma generation and its structure in which the above problems can be solved; a quantity of the target mass gasified is sufficiently small even when the initial density of target is a liquid or solid density and furthermore the x-ray conversion efficiency can be improved.